The food industry utilizes a large variety of treatments in the production of the many and diverse food products now available. Such treatments process food into the different forms and types of food products expected by the present-day consumer and also convert food into non-perishable forms, the latter requirement being well appreciated as highly desirable even by primitive man. As far as fluid foods are concerned, widely used treatments include simple mixing; emulsifying; homogenizing; comminuting; heating/cooling, and so on, and many types of devices are available for carrying out such treatments.
For example, fluid foods (or other fluid substrates) required to be emulsified, such as salad dressings, can be processed in equipment which include simple agitators utilizing mechanically-rotatable paddles or other mixing devices which provide more severe treatment, such as turbine agitators, where fixed baffles are located on the tank wall or, as in a turbine rotor and stator assembly, adjacent the propellers. The well known colloid mill is widely used to convert two or more fluids into an emulsion having a uniform droplet or particle size due to the fixed small clearance between the rotor and stator. In some instances external cooling may be provided to remove heat generated by the relatively high shearing forces applied to the emulsion. Another high shear mixing device is the homogenizer which operates by forcing the phases being processed past a spring-seated valve. However, such a treatment can result in the fine particles uncontrollably clumping up and the so called "bunches of grapes" thereby produced must then be separated by passing the fluid substrate through a second stage of the homogenizer. It will be appreciated therefore that in such circumstances the use of homogenizer apparatus necessarily entails a two-stage treatment process.
Turning to heat transfer treatments: many devices are used for this purpose including the many forms of heat exchangers which have been used for many years such as plate and falling film devices as well as the more recently developed scraped surface heat exchangers. The latter devices are widely used in the food industry, refer for example to the review article entitled "The Role of Scraped Surface Heat Exchangers in the Food Industry" by R. H. Ray in the April 1970 issue of Food Trade Review. Such devices provide a relatively large treatment zone (about 60 mm or more, depending on the size of the device) through which the product is passed, this zone being formed between the inner surface of a heat exchange tube and a rotatable shaft located within that tube. The shaft carries a number of generally radially extending scraper blades which, when the unit is in operation, continuously scrape product being processed from the inner surface in order to minimize burning on, scaling or crystalization of product on the heat exchange surface(s). Moreover the turbulent passage of the blades through the product as they are rotated about the shaft provides for some mixing of the product in order to enhance the uniformity of the treatment to which the mass of product as a whole is exposed. This type of processing is known in the engineering arts as "statistical" processing. This term is used to describe processing conditions (such as product temperature gradient, for example) which are not maintained uniformly throughout the treatment zone. Accordingly, continuous mixing of the product is made necessary in order to ensure statistically that all of the product is brought into that region of the treatment zone wherein the desired processing conditions are manifest under the given operating conditions of the specific device for the particular product in question, i.e., the "active processing zone". Clearly, only a fraction of the product contained within the treatment zone is in the active processing zone and therefore at any given moment in time subject to the intended processing conditions. The treatment of that product mass as a whole, therefore, is carried out by moving (by mixing) already treated product out of the active processing zone within the treatment zone and replacing it with untreated product from outside of that zone. The processing is therefore "statistical" in nature since the exchange of untreated for treated product in the active processing zone is largely random. Equally clear is the fact that as the time during which a given sample of product is resident within the treatment zone increases, so also does the percentage of the product in that sample which has been treated. Given the random effect of the product mixing in the treatment zone, the probability that treated product will be replaced by already treated product in the intended processing zone, also increases with time. The effect of such processing is to place a theoretical lower limit on the variance about an "ideally treated product" mean beyond which the uniformity of the product treatment cannot be improved.
In practice even that theoretical limit cannot be approached since other product flow patterns and especially eddy currents generated by the blade support struts, mean that even product residence time within the treatment zone will not be uniform. In many instances, this variation in the treatment to which product is subjected is not commercially significant in the effects that it has on the product. In other instances, however, such as where fluid containing proteinaceous materials (colloids in particular) are to be treated, the variation can be detrimental to the commercial acceptability of the resulting product. The annular space must obviously be wide enough to accommodate the scraper blades and is 60 mm or more depending on the size of the device, the active processing zone being significantly smaller than that size.
It remains only to be noted that commercially available scraped surface heat exchangers are generally designed to operate continuously at shaft rotational speeds of about 250 rpm to 300 rpm, and exceptionally up to about 500 rpm. Such devices therefore provide efficient mixing and heat transfer but only relatively moderate levels of shear.